Many metallic and semiconducting materials develop intriguing optical, electrical, and magnetic properties at the nanoscale, making them prime targets for the development of nanomaterials with novel functionalities. Such materials are often used to form nanorods. Among the potential applications of nanorods are novel surface coatings including superhydrophobic coatings and electrical and magnetic nanowires and their use as optical metamaterials for long-distance imaging with near-field resolution.
Selecting the length or aspect ratio of metallic nanorods provides a particularly straightforward method for modulating their prominent optical absorption bands. Their near infrared absorption peaks are particularly favorable for biomedical applications. The observation that anisotropic nanorods are more readily taken up into cells than their spherical counterparts has provided additional impetus for development of biomedical applications.
The reliable characterization of the physical and optical properties of nanorods, ideally in their natural solution environment, is critical for evaluating their performance in virtually all of the above-mentioned applications. Currently, the optical absorption spectrum of metallic nanorods and, in particular, the peak wavelength of the longitudinal surface plasmon resonance (LSPR), is used to derive aspect ratios for gold nanorods. However, the relationship between theoretically-predicted and experimentally-observed absorption spectra for metallic nanorods remains controversial. Counter to long-held assumptions, recent theoretical and experimental efforts indicate that the absorption peak of the surface plasmon resonance is not uniquely determined by the geometrical aspect ratio (length/diameter) of the nanorods. In addition, the optical absorption spectrum does not provide a direct measure of the heterogeneity of sizes and shapes often encountered in practice. Although electron microscopy provides an unbiased measure of particle morphologies and size distributions, it does not yield information about growth kinetics. In addition, it does not indicate whether surfactants, commonly used during synthesis or for maintaining solubility of nanoparticles, adsorb onto nanorod surfaces or whether nanorods remain dispersed or begin to interact and/or aggregate in suspension.
From the above discussion it can be appreciated that it would be desirable to have an alternative system and method for determining the dimensions of nanoparticles.